The present invention is related to methods for producing coal particles having particle sizes in the range of one micron and below. In particular, the present invention is directed to the use of swelling agents which are subsequently removed from fractured coal particles so as to further reduce their post-fracture size distribution.
The utilization of coal as an energy resource appears to be more and more desirable as the supply of other fossil fuels, such as oil, is depleted. However, the utilization of coal in various energy processes often requires the removal of mineral matter from the raw coal. If such mineral matter is not removed, corrosion, fouling, erosion and environmental hazards can be associated with certain coal utilization processes. One of the methods for removing undesirable mineral matter from coal is the grinding of coal to smaller and smaller particle sizes. This means that smaller and smaller occluded and clathrated indigenous mineral particles can be released. However, the smaller the particle size distribution that is desired, the more time and energy is required to produce such particles. Existing methods for grinding coal have reached a particle size limit of approximately one micron. Since mineral matter occurs to significant degrees in sub-micron sized coal particles, producing sub-micron sized coal particles is a necessary prerequisite to the mechanical or other removal of undesired mineral contaminants.
Several mechanical methods to grind coal presently exist. These include fluid energy mills, ball mills, stirred ball mills, hammer mills, and the like. See, for example, the "Department of Energy Coal Grinding Task Group, Final Report" by W. Bunker, W. Siskind, J. Nardella, K. Sommer, and D. Stueve, available from the Assistant Secretary for Fossil Energy, Division of Coal Utilization Systems, Washington, D.C. 20545, June, 1982. This article reviews several mechanical methods for coal comminution.
Most germane to the present invention is the utilization of fluid energy mills consisting of an annular fluid mill, a jet-vortex mill, and an opposed-jet mill configuration also reviewed in the abovementioned Final Report. In general, these mills use air, steam or nitrogen to accomplish entrainment of coal particles which in turn collide against one another and the vessel walls to fragment to particle sizes in the range of one micron and above. However, this technology is limited in that once a critical particle size is reached, the particles tend to be entrained in the exit gas stream. Furthermore, as the particle size is reduced, the probability of collision and the energy of collision and the friability of coal particles is reduced. Accordingly, it is seen that present fluid milling methods for coal comminution are inherently limited. Nonetheless, the desire to produce sub-micron sized coal particles remains.
Additionally, the use of chemical agents to accomplish coal swelling in coal comminution is also known. See, for example, U.S. Pat. No. 3,870,237; U.S. Pat. No. 3,815,826; and U.S. Pat. No. 3,850,477 all of which issued to Aldrich et al. In this respect see also the article by I. G. C. Dryden, pp. 502-8 of the 1952 issue of "Chemistry and Industry", June 7, 1952. Furthermore, the microscopic reversibility of coal swelling processes has very recently been demonstrated as is discussed in a preprint of an article for the American Chemical Society Fuel Division by D. Brenner Volume 27 (Issues 3-4), page 244 (1982). However, the use of chemical agents to accomplish coal swelling and coal comminution have not heretofore been employed for the production of sub-micron sized coal particles. More particularly, chemical swelling agents have not been employed as entrainment fluids for fluid energy mills. Accordingly, while chemical and mechanical methods have been employed to produce coal particle sizes in the range of one micron and above, they have not heretofore been employed in a cooperative manner to produce sub-micron sized coal particles from which undesirable mineral species may be more readily removed. Such removal is nonetheless highly desirable for the production of coal for use in direct coal fired diesel engines, turbines, or as chemical feedstocks.